The present invention is related to the electrochemical regeneration of composite activated carbon materials and the separation of adsorbed organic compounds and heavy metals there from. The overall process can be used for purification of liquid and gaseous media.
Activated carbon adsorbent is widely used for purification of liquid and gaseous media from organic matter. Typically, activated carbon adsorbent is used in the so-called percolation method, where liquid or gas containing organic compounds is pumped through the column containing the adsorbent.
After the activated carbon is saturated with the organic compounds, the saturated activated carbon adsorbent is either replaced with the new one (and the old adsorbent is discarded or burned) or regenerated for reuse by heating it with or without steam to high temperature (typically from 300 to 600 degrees C), whereby the adsorbed organic compounds are vaporized/destroyed. Thermal regeneration of the activated carbon adsorbents requires large energy expenditures, degradation (to 5-10%) of the activated surface during each regeneration and expensive high temperature equipment. Either chemical or electrochemical regeneration can also be used. Chemical regeneration of the activated carbon adsorbents causes degradation and blockage of the activated carbon to 10-15%.
There are known methods of electrochemical regeneration (desorption) of the activated carbon adsorbents, mainly activated carbons in granular or powdered form. U.S. Pat. No. 3,730,885 (issued May 1, 1973) describes a method of electrochemical regeneration of the activated carbon by creating a potential differential between the surface of the activated carbon adsorbent saturated with organic compounds and the desorbing solution. It describes the desorption of the described compounds from the surface of the activated carbon material by the way of polarizing activated carbon to xe2x88x921 volt (in reference to a saturated calomel electrode). Solutions for desorption were 10xe2x88x922 M Na2SO4 and 0.7xc2x710xe2x88x922 NaCl. Powdered activated carbon (an average particle size was 0.044 mm) in a mixture with Teflon dispersion (fluoroplastics) (17:3 ratio) was used as activated carbon material. U.S. Pat. No. 3,730,885 describes using currents up to 1 milliampere per gram of the activated carbon at the potential of up to 1 volt. This patent shows that changing polarity of the activated carbon material causes the desorption of strongly polarized organicsxe2x80x94acetic acid (initial adsorption capacity of the activated carbon material) during an hour at a current of less than 1 milliampere per gram of adsorbent. The attempts to desorb the adsorbed weakly polarized organic compound (amyl alcohol) resulted in that only half of the adsorbed amyl alcohol was desorbed into solution. The degree of desorption of the organic compounds adsorbed from municipal wastewater by changing polarity within the limits of 1 volt was equal to 30% (19 mg per gram of the activated carbon adsorbent was desorbed from 60 mg per gram adsorbed initially) (prototype). Thus the system of U.S. Pat. No. 3,730,885 is relatively ineffective is desorbing weekly adsorbed organic compounds.
U.S. Pat. No. 5,904,832 (issued May 18, 1999) and publications by I. V. Sheveleva et al. (xe2x80x9cRelationship between electrochemical and adsorption properties of the hydrate cellulose and polyacrylonitrile based carbon fibersxe2x80x9d Chemistry and Technology of Water, V. 12, 7, 613-616, 1990; xe2x80x9cAdsorption of phenol from water solutions by carbon fibrous electrodesxe2x80x9d, Journal of Physical Chemistry, V.64, 1, 166-169, 1990) also describe regeneration of activated carbon material that has adsorbed thereon polar/ionic organic compounds. The regeneration is done by contacting this activated carbon material with electrolyte solution, creating an electrical polarization potential on the carbon at the boundary of the carbon material and the electrolyte solution, followed by regeneration of this activated carbon material. The adsorbed organic compounds are thus transferred from the carbon adsorbent into the electrolyte solution due to their charge, and movement in the electric field. In the above publications I. V. Sheveleva describes the regeneration of the activated carbon fibers with phenol adsorbed thereon by contacting activated carbon fibers with 1 N potassium sulfate solution (pH 12) and by creating a potential from xe2x88x920.7 to xe2x88x921.3 volt.
U.S. Pat. No. 5,904,832 describes the regeneration of activated carbon adsorbent with the simultaneous destruction of the desorbed organic compounds. It was possible to regenerate activated carbon adsorbent while desorbing the adsorbed phenol removed from a waste stream. A negative potential is applied to the activated carbon adsorbent. Any type of activated carbon may be employed. The electrolyte concentration for desorption is chosen so as to avoid excessively high voltage (too much heat generation). The carbon column in U.S. Pat. No. 5,904,832 comprises metal screens inside carbon electrodes for distributing electric current inside the column. In experiments (1-16) of U.S. Pat. No. 5,904,832 by Clifford there was achieved regeneration from 30% to 80% of the phenol adsorption capacity by using currents of up to 5-10 milliamperes per gram of activated carbon adsorbent. The time of regeneration was from two hours for regeneration of less than 50% to 45 hours for 80% regeneration.
The aforesaid prior art systems displace the adsorption equilibrium by polarizing the boundary between carbon adsorbentxe2x80x94solution. In this case the drop of the potential at the cell is several volts (mostly, less than a volt), the currentsxe2x80x94from 1 to 10 milliamperes per gram of adsorbent. It is only possible to shift the equilibrium significantly by means of polarization for compounds that are ionic form in one or another range of pH: phenols, sulfosalicilic acid, organic bases. This is the reason why all examples in these reference descriptions are based on these compounds.
The above described methods of regenerating carbon adsorbents with the organic matter adsorbed thereon by polarization have not been commercialized due to a number of drawbacks:
relatively efficient regeneration (over 50%) of the adsorbed organic matter was achieved only for ionic (strongly polarized) organic pollutants (acetic acid, phenol). Regeneration took place due to electrostatic (ionic) repulsion of the charged organic molecules from the same charged surface of the activated carbon adsorbent (electrode).
at these conditions only 50-90% regeneration for phenol was achieved.
regeneration required a long time (from several to 45 hours).
The present invention teaches a new improved electrochemical process for desorbing adsorbed materials (nonpolar and polar organic compounds and heavy metal ions) from carbon adsorbents. The time for regeneration is decreased, and the degree of regeneration increased. Multiple regenerations may be employed.
For achieving a high degree regeneration and adsorption it is necessary that:
The composite adsorption regenerable carbon material (CRAC) has sufficiently high electric conductivity for uniform potential distributionxe2x80x94a volumetric electric conductivity of 1-100 (Ohm.m)xe2x88x921 
The adsorbent (CRAC) specific volumetric electric must differ from the specific electric conductivity of electrolyte which fills up the pores of the adsorbent by not more than an order of magnitude (if the specific electric conductivity of the adsorbent (CRAC) is much higher, the current will flow preferably through the adsorbent, and if the specific electric conductivity of the electrolyte is much higher, the current will flow through the electrolyte),
The specific current density has to be at least 0.01 ampere per gram of CRAC (so as to provide a uniform current flow through the surface of the adsorbent-electrolyte solution interface). When there is such high current flow, the surface of the adsorbent becomes highly hydrophilic due to the discharges of the ionic particles taking place at the adsorbent""s surface. As a result the affinity of the surface towards organic (including nonionic, for example, chloroform) compounds becomes sharply lower, leading to their desorption,
The adsorbing particles should preferably have large outer surface and large, developed macro- and microporous adsorption surface, due to their small diameter (less than 30 micron) and large adsorption surface area (over 500 m2 per gram as per nitrogen). As a result, the material has large share of the adsorptional centers at the surface which are subject to the direct influence of the electric current, and, also, quick diffusion of the desorbed matter into solution. The porous matrix should preferably have a specific volumetric electric conductivity of 1-100 (ohm.m)xe2x88x921.
Preferably the composite regenerable adsorption carbon material comprises (as the activated carbon particles of the porous matrix) activated carbon fibers at least 1 mm long, 1 to 30 micron in diameter, with a methylene blue adsorption capacity of at least 200 mg per gram and the adsorption surface area of at least 500 m2 per gram. The composite regenerable adsorption carbon material comprises carbon fibers at least 2 mm long, 1 to 30 micron in diameter. As the ion-exchange material it comprises ion-exchange fibers 1 to 30 microns in diameter with the exchange capacity at least 1 meq per gram or ion-exchange resins.
As the regeneration method the electric current may be passed through the porous matrix at a specific density preferably of at least 0.05 ampere per gram of the composite adsorption carbon material. The adsorbent layer is filled with the electrolyte water solution periodically during regeneration or after regeneration or continuously during regeneration. The adsorbed thereon compounds comprising organic compounds, including nonpolar, polar, ionic, biological, including bacteria, and heavy metal ions. The electric current may be applied in at least two stages with the polarity change, by passing through cathode or anode current.
Experimentally obtained linear relationship of the amount of the washed off (desorbed) matter and time at the constant electrolysis current, or linear dependence on the amount of electricity (FIG. 1) confirms the role of the current transfer through the interface boundary for desorbing organic compounds and heavy metals as compared to the prototype where the surface polarization takes place.
In contrast to U.S. Pat. No. 3,730,885) where forms a specific medium (acidic) for transferring species into the ionic forms for desorption, in the present invention desorption takes place due to the passage of the electric current, which affects all adsorbed particles including nonpolar ones.
The claimed method can be applied for the regeneration of the composite adsorption carbon material not only from the ionic, but also from polar and nonpolar organic substances (benzene, chloroform) as well as heavy metals.
Materials, well known for purification from organic compounds and heavy metals, were used for the composite regenerable adsorption carbon material, which forms the electrode.
Activated carbon fibers, manufactured by Aquaphor Corp., St.Petersburg, Russia, were at least 1 mm long, 1 to 30 micron in diameter, with the adsorption capacity for methylene blue at least 200 mg/g, and the adsorption surface area at least 500 m2/g. Activated carbon fibers were obtained by pyrolysis and carbonization of viscose fibers (U.S. Pat. No. 5,521,008) followed by steam activation. Depending on conditions of carbonization and activation there may be obtained activated carbon fibers with different specific electric conductivities in the layer within the limits from 1 to 100 (Ohm.m)xe2x88x921.
Nonactivated carbon fibers at least 2 mm long and 1 to 30 micron in diameter were obtained by pyrolysis and carbonization with the final carbonization temperature more than 800xc2x0 C. Specific electric conductivity of a layer of carbon fibers is xcx9c100 (Ohm.m)xe2x88x921.
Ion-exchange materials in a form of fibers of 1 to 30 micron in diameter were obtained by partial hydrolisis of polyacrylonitrile fibers in presence of the binding agent, so that the exchange capacity was at least 1 meq/g and used as granulated ion-exchange material was e.g. sulphocationite C240NS produced by Sybron Chemicals Inc, USA.
Granulated activated carbon is porous carbon particles of variable size of granules manufactured by Barnebey and Sutcliffe Corp.
The composite regenerable adsorption carbon (CRAC) material by the claimed invention was obtained by mechanical mixing of components.
Additionally, it may be obtained by manufacturing a porous matrix containing activated carbon particles with polymeric binding and its subsequent carbonization, so that it attains the desired specific volumetric electric conductivity.
Water solutions of sodium sulphate, sodium carbonate and sodium chloride may be used as electrolytes in the electrochemical cell of the present invention. The electrolytes were selected by the specific volumetric electric conductivity as compared with that of their composite activated carbon material.
The second electrode is made as inert electrode of graphite (e.g. in form of a graphite rod).
Used in the electrochemical cell design were materials inert to adsorbed substances.
The composite adsorption carbon material may be regenerated both at least in one electrochemical cell or in several cells engaged in parallel, whereby the regeneration in one cell may proceed irrespectively of other electrochemical cells.